Betta mahachaiensis, a new species of fighting fish belonging to the Betta splendens group, is described. The fish inhabits specifically sites with brackish water and nipa palms in the Samut Sakhon Province, Thailand. It is distinguishable from other members of the B. splendens group in having an iridescent green/bluish-green appearance on the brown-to-black body background. The opercular membrane has brown-to-black colour without red streaks or patches. The opercle has two parallel iridescent green or bluish-green vertical bars. Its dorsal, caudal, and anal fin rays are brown to black, contrasting with the iridescent green or bluish-green of the interradial membrane. Black transverse bars are present on at least the proximal twothirds of the dorsal fin, but not on the caudal fin. The colour of the pelvic fins is brown to black with an iridescent green/bluish-green front margin and a white tip. Females possess similar characters but are less colourful than males.

FIGURE 5. (a) Map of Thailand showing Betta mahachaiensis catch sites (encompassed by the small rectangle in the middle) in the central part of Thailand (C-Central, N-North, S-South, W-West, E-East, and NE-Northeast); (b) Catch (stars) of Betta mahachaiensis between years 2007 and 2012, twenty-eight sites in Samut Sakhon province and two sites in west of Bangkok.

FIGURE 6. Habitat of Betta mahachaiensis in Samut Sakhon province, Thailand: (a) Nipa palm forest behind a food factory (Type Locality) (30 Apr. 2011); (b) Water held in the pocket phytotelma of the nipa palm bract as spawning space and the fish (THNHM-F-01632) caught from it (30 Apr. 2011); (c) The fish (THNHM-F-01655) and bubble nest in a pocket of a nipa palm bract (29 May 2011).